Organic & Biomolecular Chemistry Blog

It is next to impossible to achieve good mixing with traditional magnetic stir bars in the cylindrical vessels used in microwave synthesis. So scientists in Austria have designed a new stir bar. 

Microwave synthesis is becoming more popular thanks to the dramatically reduced reactions times and improved yields it offers. However, as the majority of these reactions involve a small narrow reaction vessel, traditional horizontal stir bars often result in inefficient mixing which can cause temperature gradients to develop and reduce product yields. 

Magnetic stirring usually involves a polymer coated AlNiCo or ferrite magnet and relies on the establishment of a liquid flow induced by centrifugal forces. These flow patterns involve a downward liquid motion towards the centre and an upward motion at the vessel wall. However, the strength of the magnet decreases rapidly over a period of months when subjected to the high temperatures of microwave synthesis and the limited size of the vessel restricts the establishment of flow patterns. 

‘We noticed that in some instances the agitation of the reaction mixture using a standard magnetic stir bar was very poor. Sometimes the stirrer was not moving at all,’ says Oliver Kappe whose team at the University of Graz used a camera to look at a microwave system before designing the new stir bar. 

Choice of magnetic material and shape were thought to be key to the success of the new stir bar. Vertical blade extensions were added to a more robust Sm₂Co₁₇ rare earth magnet. This not only extends the life of the stir bar but also significantly enlarges the cross-sectional area of the stirrer. A gap in the centre of the bar guarantees backflow which enforces flow patterns and extended blades mean that even the upper part of the system receives efficient mixing with the incorporation of additional holes and cut-outs inducing additional turbulence.

Read the full story in Chemistry World 

Design and evaluation of improved magnetic stir bars for single-mode microwave reactor
David Obermayer, Markus Damm and C. Oliver Kappe
Org. Biomol. Chem., 2013, Accepted Manuscript
DOI: 10.1039/C3OB40790J

Imagine finally completing a 30 step total synthesis only to discover that the molecule you were aiming for was the wrong one. The consequences of structural misassignment of complex organic molecules can be costly and time consuming, not to mention frustrating. Now, a new NMR method aims to highlight errors in proposed structures at a much earlier stage, preventing such scenarios.

NMR spectroscopy is a standard tool for elucidating the structure of organic molecules. This may be a straightforward job when confirming the identity of small molecules. However, in the case of complex molecules, the task becomes much more difficult and errors can result in the wrong structure being proposed.

Ariel Sarotti from the Rosario National University, Argentina, has developed a new, computationally inexpensive method combining calculated and experimental ¹³C NMR data to flag up incorrect structures. This rapid and simple process can determine if a candidate structure is incorrect, using trained artificial neural networks (ANNs) to find patterns in both the calculated and experimental data to do the decision making. A set of 200 molecules with known correct and incorrect NMR assignments was used to create and train the system. The subsequent testing phase correctly identified the incorrect structures of a set of 26 natural products. While some knowledge of computational chemistry is required, Sarotti’s development of an Excel spreadsheet tool will allow chemists to use the method without being experts in ANNs, making it much more accessible.

Read the full story in Chemistry World

Successful combination of computationally inexpensive GIAO 13C NMR calculations and artificial neural network pattern recognition: a new strategy for simple and rapid detection of structural misassignments
DOI: 10.1039/C3OB40843D